Code division multiple access (CDMA) communication systems are used extensively in satellite communications with military and commercial applications. These systems are also known as spread spectrum communication systems because the communicated information is spread over a wide allocated frequency spectrum. In CDMA communication systems the frequency spectrum can be reused multiple times.
Because CDMA modulation techniques are inherently more susceptible to fading conditions present at the terrestrial and land mobile environments, their application has been limited to satellite communications. However, with recent advancements in communication signal processing, CDMA communication systems are becoming increasingly popular in terrestrial land mobile communication environments as well. For example, recent developments have allowed CDMA systems to be used in cellular telephone communications environments.
In general, there are two CDMA types of communication systems. One is known as frequency hoping CDMA system where the wide allocated spectrum is divide into a substantial number of narrower frequency band and information signal is switched or "hoped" over these frequency bands in accordance with a predetermined code. The other CDMA system is known as a direct sequence CDMA communication system (DS-CDMA) where the user information signals in the form of binary bits are spread over the allocated frequency spectrum by combining them with spreading codes known as pseudorandom noise (PN) codes. The spreading code comprises a predetermined sequence of binary states known as chips. Thus, when combined, each user information bit interval gets coded with a spreading chip sequence. Conventionally, a DS-CDMA transmitter produces a direct sequence spread spectrum (DS-SS) communication signal by multiplying the user information bit sequences by the spreading chip sequence.
Once received at a receiving end, the DS-SS communication signal is decoded by multiplying the received signal by a despreading chip sequence having corresponding characteristics to the spreading chip sequence. In conventional DS-CDMA communication system, the receiver knows of the spreading chip sequence prior to start of a communication call. Thereafter, the receiver decodes the DS-SS communication signal based on the known spreading chip sequence.
It is well known that in the presence of many users CDMA receivers in addition to receiving the desired signal also receive many multiple-access interfering signals. In presence of multiple access interference, reliable communication may be achieved when interfering signals are received at approximately the same power level. When, there is a large disparity in received signal powers, non-zero crosscorrelations among the signals gives rises to a phenomenon known as near-far problem. In near-far situations, higher power interfering signals significantly degrade reception and decoding of a lower power desired transmission.
One conventional approach to improving the near far problem uses a power control scheme where the powers from the receivers are fed back and transmitter powers are controlled to substantially remove the power disparity. In another approach, PN codes are constructed such that they provide orthogonality between the user codes, thereby reducing mutual interference. This allows for higher capacity and better link performance. With orthogonal PN codes crosscorrelation is zero over a predetermined time interval resulting in no interference between the orthogonal codes provided only that the code time frames are aligned with each other.
In conventional CDMA communication systems the spreading chip sequence is either assigned by a self controller or it is pre-stored within the receiving unit. As such, during despreading and demodulation process, the receiver knows of the spreading chip sequence. A more recent approach for a CDMA receiver proposes an adaptive despreading or demodulating process. In an adaptive CDMA system, the receiver is enabled to suppress multiple access interference by an adaptive equalization process. In such a system, a CDMA transmitter transmits a training bit sequence which is coded with the spreading chip sequence and the receiver adaptively determines, based on the training sequence, the despreading code using a tapped delay line equalizer. Adaptive determination of the despreading chip sequence and suppression of multiple access interference allows significant number of users to communicate with each other over an spread spectrum channel without requiring central control infrastructure, and as such paving the way for infrastructureless communication systems.
However, in adaptive CDMA communication, the determined despreading chip sequence is not time synchronized with the transmitter because of certain time delays within the communication path or simply because the receiver does not know when bit and chip timing of the transmitter starts. Conventional methods of determining bit timing and chip timing offsets between the transmitter and receiver comprise performing correlation routines involving complex mathematical processing operations. These operations are time consuming and therefore delay establishment of communication link between transmitter and receiver. Therefore, there exists a need for a faster synchronization method which could be achieved in significantly shorter period of time than is achievable by conventional methods.